Systems and methods of thermal energy storage

ABSTRACT

Systems and methods of energy storage and release comprise at least one storage vessel and a combined conveyor and heat transfer device linked to the at least one storage vessel by at least one discharge device. The combined conveyor and heat transfer device includes a rotatable conveyor drum and at least one heat transfer fluid conduit within the rotatable conveyor drum. A granular material travels from the at least one storage vessel to the combined conveyor and heat transfer device via the at least one discharge device, and the rotatable conveyor drum moves the granular material therethrough in counterflow to a flow of heat transfer fluid traveling through the heat transfer fluid conduit.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent ApplicationSer. No. 61/241,909, filed Sep. 13, 2009, which is incorporated byreference herein in its entirety.

FIELD

The present disclosure is in the technical field of Thermal EnergyStorage (TES).

BACKGROUND

In concentrated solar power (CSP) or similar energy systems, heattransfer fluid (HTF) is used to convey energy from the heat source toand/or from the energy conversion or use system. In CSP systems, theheat source is an array of concentrating solar collectors, and theenergy conversion system is typically a heat engine such as a steamcycle or organic Rankine cycle. In CSP systems in particular, thefunctionality and, potentially, the economic worth of the system isenhanced by (TES). The benefit of TES comes from extending the operatingtime of the energy conversion system or shifting the time of energyproduction to a more favorable time when energy is more valuable.

Various TES technologies have been developed, particularly for CSPapplications, including the two-tank TES system and the single-tankthermocline, both of which have direct and indirect variations(referring to whether the HTF and thermal storage medium are the same orare segregated and interfaced through a heat exchanger). Each of thesetechnologies has pros and cons related to system cost effectiveness,commercial history, and operational attributes. For example, a two-tanksystem using HTF with high vapor pressure requires plants at hightemperature HTFs requires costly pressurized storage tanks. Systems withmolten salt varieties as a HTF and/or thermal storage media requirespecialized tanks and heat exchanger designs. The single-tankthermocline can be a cheaper option due to reduced capital costs, yetmust consider the same issues with the type of HTF used. In general,current TES technologies require heat transfer fluids and thermalstorage mediums with significant cost and design implications.

Thus, there is a need for thermal energy storage systems and methodsthat can effectively use inexpensive materials as a storage medium andare compatible with a variety of heat transfer fluids.

SUMMARY

Embodiments of the present disclosure provide alternatives to, andalleviate many of the disadvantages of TES systems by providing thermalenergy storage devices, systems and methods which utilize granularmaterials as a thermal energy storage medium that is compatible with avariety of HTFs. Exemplary embodiments include a heat exchanger that iscomprised of an Archimedes screw conveyor design to transport sand overan internal HTF tube bundle, which contains heat transfer fluid used tostore and remove heat from the sand. Embodiments of the presentdisclosure effectively use sand, a relatively inexpensive andenvironmentally benign material, as a thermal storage medium while alsoproviding heat transfer and heat exchange capabilities. Alternativegranular materials would include other high temperature tolerantparticles.

Advantages of the disclosed systems and methods include, but are notlimited to: (1) use of sand or other inexpensive and inert granularmaterial as the storage medium, which is environmentally benign,inexpensive, non-volatile, acceptable in thermal properties, (2)delivery of a constant temperature heat from the silos since a constanttemperature will be maintained in the bins irrespective of current sandvolume, (3) compatibility with a variety of HTF fluids, as the design isworking-fluid independent, (4) achievement of high “round trip thermalefficiency” since energy loss is minimal, and (5) applicability to otherCSP technology and other thermal systems.

In exemplary embodiments a system of energy storage and releasecomprises at least one storage vessel and a combined conveyor and heattransfer device linked to the at least one storage vessel by at leastone discharge device. The combined conveyor and heat transfer deviceincludes a rotatable conveyor drum and at least one heat transfer fluidconduit within the rotatable conveyor drum. A granular material travelsfrom the at least one storage vessel to the combined conveyor and heattransfer device via the at least one discharge device. The rotatableconveyor drum moves the granular material therethrough in counterflow toa flow of heat transfer fluid traveling through the heat transfer fluidconduit. In exemplary embodiments the granular material is sand.

In exemplary embodiments, the rotatable conveyor drum may be anArchimedes screw and may comprise one or more vanes fixed to an innersurface of the drum. The one or more vanes may be spiral shaped,longitudinally straight, substantially T-shaped or substantiallyV-shaped in cross-section to distribute the granular material over theheat transfer fluid conduits. The at least one heat transfer fluidconduit may comprise a plurality of tubes arranged in a bundle. Inexemplary embodiments, when the rotatable conveyor drum rotates thegranular material pours over the at least one heat transfer fluidconduit such that heat exchange occurs between the granular material andthe heat transfer fluid. The one or more vanes may pick up and rain thegranular material over the at least one heat transfer fluid conduit.

In exemplary embodiments, the at least one storage vessel comprises afirst and second storage vessel, and the first storage vessel has ahigher temperature than the second storage vessel. The at least onestorage vessel may be located above or below ground level and may haveat least one angled wall. In exemplary embodiments, the stored energy isheat gathered by, or discharged to, a concentrating solar thermal powerplant.

Exemplary embodiments include methods of storing thermal energy.Exemplary methods comprise providing a granular material and a heattransfer fluid. The heat transfer fluid has a temperature relativelyhigher than a temperature of the granular material. The granularmaterial and the heat transfer fluid are conveyed such that the granularmaterial continually pours over a tube carrying the heat transfer fluidsuch that heat exchange occurs between the granular material and theheat transfer fluid. A set of vanes may direct the pouring of theconveyed granular material, and the granular material may be sand. Thegranular material may travel in overall counterflow to a flow of heattransfer fluid or in overall cocurrent flow to the flow of heat transferfluid.

Exemplary methods may further include methods of releasing storedthermal energy comprising providing a granular material and a heattransfer fluid. The granular material has a temperature relativelyhigher than a temperature of the heat transfer fluid. The granularmaterial and the heat transfer fluid are conveyed such that the granularmaterial pours over a tube carrying the heat transfer fluid such thatheat exchange occurs between the granular material and the heat transferfluid.

In exemplary embodiments, a combined conveyor and heat transfer devicecomprises a rotatable conveyor drum and at least one heat transfer fluidconduit within the rotatable conveyor drum. The rotatable conveyor drummoves a granular material therethrough in counterflow to a flow of heattransfer fluid traveling through the heat transfer fluid conduit. Therotatable conveyor drum may be an Archimedes screw. When the rotatableconveyor drum rotates, the granular material pours over the at least oneheat transfer fluid conduit such that heat exchange occurs between thegranular material and the heat transfer fluid. The rotatable conveyordrum may be capable of rotating at one or more speeds.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an embodiment of the present disclosure,showing a thermal energy storage and release system with above groundstorage vessels containing sand thermal storage medium;

FIG. 2 is a front view of an embodiment of a rotatable conveyor drumcontaining the heat transfer tube bundle in accordance with the presentdisclosure;

FIG. 3A is a side view of an embodiment of a rotatable conveyor drumcontaining an embodiment of a heat transfer tube bundle in accordancewith the present disclosure;

FIG. 3B is a side view of an embodiment of a rotatable conveyor drumcontaining an embodiment of a heat transfer tube bundle in accordancewith the present disclosure;

FIG. 4 is a top view of an embodiment of a supply and return pipingarrangement for a heat transfer tube bundle in accordance with thepresent disclosure;

FIG. 5 is a perspective view of an embodiment of a thermal energystorage and release system with in-ground storage vessels containingsand thermal storage medium in accordance with the present disclosure;

FIG. 6 is a side view of an embodiment of a thermal energy storage andrelease system with in-ground storage vessels during the thermal energystorage charging process in accordance with the present disclosure;

FIG. 7 is a side view of an embodiment of a thermal energy storage andrelease system with in-ground storage vessels during the thermal energydischarge process in accordance with the present disclosure; and

FIG. 8 is a side view of an embodiment of a storage vessel of a thermalenergy storage and release system in accordance with the presentdisclosure.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the invention in more detail, FIG. 1 depicts anexemplary embodiment of a thermal energy storage system (sometimesreferred to herein as “sand-shifter”) that uses a very inexpensive andbenign storage medium: sand or similar granular material. Exemplaryembodiments of a sand-shifter thermal energy storage system consist of ahigher temperature above ground vessel 2 as well as a lower temperatureabove-ground storage vessel 3, each filled with sand to function as thethermal energy storage medium. The lower temperature above groundstorage vessel 3 contains only moderately warm sand that is available tobe heated and store energy, and the higher temperature above groundstorage vessel 2 contains hot sand after it has been heated to storeenergy.

As shown in FIGS. 1-3B, a system of energy storage and release 30includes a combined conveyor and heat transfer device 40 comprising aconveyor 1, which may be an Archimedes screw conveyor and heat transferfluid inner heat transfer tube bundle 8. In exemplary embodiments, thesand or similar granular material is moved by the conveyor in adirection 35 roughly in counterflow to the flow 38 of the heat transferfluid 28, but cocurrent flow may also be employed. The HTF 28 will becirculated through a heat transfer tube bundle to contact the sandwithin the conveyor 1. The heat transfer tube bundle 8 may be one ofvarious designs including a tube bundle of pipes, bare tubes, finnedtubes, and/or plate heat exchangers; where the design consists of thebasic concept of effectively transporting heat transfer fluid throughthe conveyor 1 to come into contact with the sand thermal energy storagemedium. FIG. 1 provides an illustration of how the HTF heat transfertube bundle 8 may be configured within the conveyor 1, which will poursand or similar granular material 15 over the HTF 28 to adsorb or giveup heat depending on whether the sand is being heated or cooled. Sand orother granular material enters the conveyor 1 via the horizontaldischarge augers (or other suitable conveyor) 9, 10, and sand isrecovered to the top of the storage vessels via vertical conveyors 6, 7.

It is understood that alternatively the granular material might be movedbetween the top and the bottom of a single vessel. It is also understoodthat the heat transfer tube bundle may employ finned tubes to promoteheat transfer and distribution of the sand.

As shown in FIG. 1, when energy is being stored the lower temperaturesand is removed from the lower temperature above ground storage vessel3, heated by the heat transfer tube bundle 8 containing highertemperature HTF 28 from the solar collector field 4, 5 and transferredto the higher temperature above ground storage vessel 2. When energystorage is complete, the higher temperature above ground storage vessel2 will be largely full of hot sand. When stored energy is needed, hotsand will be returned to the lower temperature above ground storagevessel 3 while stored heat in the sand is recovered by warm outlet HTF.The practical design shown allows free expansion and contraction of themetal parts to account for thermal expansion.

An exemplary conveyor used to move the sand is a variation of anArchimedes screw. The Archimedes screw is normally used as a type oflift pump. In this case, it is used as a sand conveyor and heatexchanger. As more specifically shown in FIGS. 2 and 3A-3B, theArchimedes screw conveyor 1 is a rotating sand conveyor drum 11 with oneor more spiral vanes 12 fixed to the inner surface of the drum. As thedrum turns, the spiral vane 12 pushes the sand 15 along the bottom ofthe rotating drum 11. The Archimedes screw has no close sliding fits toachieve this pushing motion; indeed, there is no sliding metal-to-metalcontact at all. As the sand 15 is conveyed by the spiral vane 12, a setof longitudinal straight vanes 13 acts to simultaneously lift and conveythe sand 15 over the heat transfer tube bundle 8 containing the heattransfer fluid (HTF) 28. By this action the HTF 28 flowing in the tubes8 is made to either adsorb or give up heat. As shown in FIG. 1, verticalconveyors 6, 7 will top-load each storage vessel, and horizontaldischarge augers or other conveyors 9, 10 will unload them from below.

The Archimedes screw sand conveyor 1 has the great advantage thatswitching the direction of rotation changes the direction of the motionof the sand. This feature makes it is easy to change the direction ofthe motion of the sand as the system is switched between the heatstorage function and the heat recovery function.

Details of the Archimedes screw conveyor 1 are shown in FIGS. 2 and 3.The spiral vane 12 (the “screw” of the Archimedes screw) is shownattached to the interior of a drum 11. Inside the drum 11, is shown aheat transfer tube bundle 8 containing the HTF 28. As the drum 11rotates, sand or other granular material 15 is pushed along laterally bythe screw spiral vane 12. An advantageous feature is that the drum alsocarries a series of longitudinal vanes 13 that pick up and rain the sand15 over the tube bundle, thus providing heat exchange as the sand 15 isconveyed (FIG. 3). Note again that the Archimedes screw has no closesliding fits and no sliding metal to metal contact at all, which is incontrast to an auger or screw conveyor. As the HTF 28 passes through theheat transfer tube bundle 8, the sand 15 pours over the pipes, eithercharging the HTF 28 with heat from hot sand or, conversely, charging thesand 15 with heat from the hot HTF 28. The tubes may be equipped withlongitudinal or transverse fins to increase the outside heat-transferarea and to retard and redirect the fall of the conveyed sand 15providing adequate time for heat transfer to or from the sand. In thisapplication, the function of the tubes and fins is analogous to theaction of the so-called “fill” in a cooling tower or packed column. Therotating drum 11 will be insulated 14 to avoid heat losses.

Various types of extended surfaces such as longitudinal, latitudinal,and/or corrugated fins may be used to increase the heat transfer surfaceon the sand side. Furthermore, the fins may have additional features toimprove the contact between the flowing sand and the base tubes. Inaddition the tubes may have elongated or elliptical shapes to improvethe contact and heat transfer with the sand. Indeed, the preferred“tube” cross section may be more plate like or similar to an elongatedrectangular passage than a generally circular “tube”. These additionalfeatures enhance the contact between tube and fins with the sand andheat transfer to or from the sand may be included

Various additional features to enhance heat transfer to or from the sandor from the tube to the internal heat transfer fluid may be included. Insome situations, for example, it may be advantageous for the granularmaterial to travel in overall cocurrent flow to the flow of internalheat transfer fluid. FIG. 3B illustrates an exemplary embodiment inwhich the sand or similar granular material 15 is moved by the conveyorin a direction 35 roughly in cocurrent flow to the flow 38 of the heattransfer fluid 28.

An overhead view of the supply and return piping, including the heattransfer tube bundle 8 is shown in FIG. 4. The heat transfer tube bundle8 is integrated into the plant via inlet and outlet large central largediameter pipes 16, 17. These pipes are connected to the heat transfertube bundle 8 through left and right hand large diameter pipes 18, 19,which are in turn connected to left and right hand plenums 20, 21. Theplenums 20, 21 handle the transfer of HTF 28 between the large diameterpipes and the heat transfer tube bundle 8. It should be noted that thestructural outriggers support the heat transfer tube bundle and bothplenums at each end. This allows free expansion from the central supportto account for thermal expansion.

It may be further understood that the option exists for the sand-shiftersystem to employ in ground storage vessels or pits as the storage volumeas opposed to above ground storage vessels. FIG. 5 shows a perspectiveview of the sand-shifter thermal energy storage system with an in groundstorage vessel setup. The main difference in this system is the need forvertical conveyors 24, 25 to transport sand out of the highertemperature in ground storage vessel 22 and lower temperature in groundstorage vessel 23.

Embodiments of charging processes to store thermal energy in the sandare shown by a side view in FIG. 6. This process will be largely similarregardless if practiced with in ground or above ground storage vessels.Higher temperature heat transfer fluid inlet from the solar collectorfield 5 flows into the sand-shifter system by way of an inlet largediameter supply pipe 16. Next the hot HTF 28 flows through the left handlarge diameter pipe (LHLDP) 18 into the left hand plenum (LHP) 20. Inthe LHP 20, hot HTF is distributed to the heat transfer inner flow core8, and flows to the right in counterflow to the conveyed sand 15. Thelower temperature sand is lifted out of the lower temperature in groundstorage vessel 23 via a vertical conveyor 24 and enters the Archimedesscrew sand conveyor 1 from a horizontal discharge auger 27 or othersuitable conveyor. Heat transfer fluid 28 exchanges heat with conveyedsand 15 roughly in counterflow until it reaches right hand plenum (RHP)21. HTF flows are combined in the RHP 21 and directed into the righthand large diameter pipe (RHLDP) 19, which is now used as the returnpipeline. The conveyed sand 15 exits the Archimedes screw and entersstorage vessel 22 view auger 26. Warm HTF in the RHLDP 19 returns to theoutlet central large diameter pipe 17 and exits the Thermal EnergyStorage system.

Embodiments of Discharging Processes to release stored thermal energyand heat the HTF are shown by a side view in FIG. 7. Again, this processwill be largely similar regardless if practiced with in ground or aboveground storage vessels. The operation of heating the HTF 28 isaccomplished by using hot sand stored in the higher temperature inground storage vessel 22. Warm HTF 28 flows in by way of an inlet largediameter supply pipe 16 and then flows in the right hand large diameterpipe (RHLDP) 19 into the right hand plenum (RHP) 21. In the RHP 21, warmHTF 28, is distributed to multiple pipes in the heat transfer tubebundle 8 and flows to the left in counterflow to the conveyed sand 15.The hot sand is conveyed 15 from the higher temperature in groundstorage vessel 22 into the Archimedes screw conveyor 1 via verticalconveyor 24 and horizontal discharge auger 26 or other suitable conveyorand exchanges heat with the warm HTF roughly in counterflow. When thewarm sand reaches the end of the Archimedes screw conveyor 1, it isreturned to the lower temperature in ground storage vessel 23 where itawaits the Charging Process. The now hot HTF reaches the left handplenum (LHP) 20 and is directed into the left hand large diameter pipe18, now used as the return pipeline. Hot HTF then returns to the outletlarge diameter supply pipe 17 and exits the Thermal Energy Storagesystem to produce usable energy.

Turning to FIG. 8, exemplary embodiments of storage vessels 102, 103 areillustrated in greater detail. The storage vessels 102, 103 may haveangled walls 132 to facilitate flow of granular materials 15 into andout of the storage vessels during operation. In exemplary embodiments,the wall angle may be about 30° or greater. This advantageousconfiguration can be employed in either above ground or in-groundstorage vessels.

In concentrator solar thermal power, embodiments of the disclosedsystems and methods are used to store heat gathered during the day thatis not needed for power generation or that is in excess of the heatneeded for power generation at some time. This heat will be stored andused to generate power when needed, such as during afternoon peakingperiods, or during the evening and nighttime. The basic concept of thesand shifter may be applicable in other applications in power generationcycles, in materials processing, or in other heat and mass transferapplications.

It should be understood that good heat transfer performance is obtainedby raining the sand 15 over a heat transfer tube bundle 8 carrying theHTF used to convey heat alternatively from the collector field or to apower conversion plant. Ideally, heat transfer coefficients moderatelyapproximating the performance seen in similarly-agitated fluidized bedswill be achieved. Good heat exchange effectiveness means close approachof the thermal storage medium to the inlet temperature of the HTF duringcharging of the storage and close approach of the HTF temperature to themaximum temperature of the storage medium during discharge. This goodeffectiveness will be obtained by heating sand or alternatively removingheat from the sand while moving the sand to or from a higher temperatureabove ground storage vessel 2 in a novel conveyor that doubles as acounter flow heat exchanger. The counter flow arrangement promotes higheffectiveness. The sand storage containers will be simple andinexpensive insulated silos or bins above ground or buried pits.

Thus, it is seen that systems and methods of storing and releasingthermal energy are provided. It should be understood that any of theforegoing configurations and specialized components or chemicalcompounds may be interchangeably used with any of the systems of thepreceding embodiments. Although illustrative embodiments of the presentinvention are described hereinabove, it will be evident to one skilledin the art that various changes and modifications may be made thereinwithout departing from the invention. It is intended in the appendedclaims to cover all such changes and modifications that fall within thetrue spirit and scope of the invention.

1. A system of energy storage and release, comprising: at least onestorage vessel; a combined conveyor and heat transfer device linked tothe at least one storage vessel by at least one discharge device, thecombined conveyor and heat transfer device including: a rotatableconveyor drum; and at least one heat transfer fluid conduit within therotatable conveyor drum; wherein a granular material travels from the atleast one storage vessel to the combined conveyor and heat transferdevice via the at least one discharge device, and the rotatable conveyordrum moves the granular material therethrough in counterflow to a flowof heat transfer fluid traveling through the heat transfer fluidconduit.
 2. The system of claim 1 wherein the granular material is sand.3. The system of claim 1 wherein the rotatable conveyor drum is anArchimedes screw.
 4. The system of claim 1 wherein the rotatableconveyor drum comprises one or more vanes fixed to an inner surfacethereof.
 5. The system of claim 1 wherein the at least one storagevessel has at least one angled wall.
 6. The system of claim 1 whereinthe at least one heat transfer fluid conduit comprises a plurality offluid conduits arranged in a bundle.
 7. The system of claim 1 whereinwhen the rotatable conveyor drum rotates the granular material poursover the at least one heat transfer fluid conduit such that heatexchange occurs between the granular material and the heat transferfluid.
 8. The system of claim 4 wherein the rotatable conveyor drumrotates such that the one or more vanes pick up and rain the granularmaterial over the at least one heat transfer fluid conduit.
 9. Thesystem of claim 1 wherein the at least one storage vessel comprises afirst and second storage vessel, the first storage vessel having ahigher temperature than the second storage vessel.
 10. The system ofclaim 1 wherein the stored energy is heat gathered by, or discharged to,a concentrating solar thermal power plant.
 11. The system of claim 4wherein the one or more vanes are one of: spiral shaped, longitudinallystraight, substantially T-shaped and substantially V-shaped.
 12. Amethod of storing thermal energy, comprising: providing a granularmaterial and a heat transfer fluid, the heat transfer fluid having atemperature relatively higher than a temperature of the granularmaterial; conveying the granular material and the heat transfer fluidsuch that the granular material pours over a tube carrying the heattransfer fluid such that heat exchange occurs between the granularmaterial and the heat transfer fluid.
 13. The method of claim 12 whereinthe granular material travels in overall counterflow to a flow of heattransfer fluid.
 14. The method of claim 12 wherein the granular materialtravels in overall cocurrent flow to a flow of heat transfer fluid. 15.The method of claim 12 wherein the granular material is sand.
 16. Themethod of claim 12 further including releasing stored thermal energy,comprising: providing a granular material and a heat transfer fluid, thegranular material having a temperature relatively higher than atemperature of the heat transfer fluid; conveying the granular materialand the heat transfer fluid such that the granular material pours over atube carrying the heat transfer fluid such that heat exchange occursbetween the granular material and the heat transfer fluid.
 17. Acombined conveyor and heat transfer device, comprising: a rotatableconveyor drum; and at least one heat transfer fluid conduit within therotatable conveyor drum; wherein the rotatable conveyor drum moves agranular material therethrough in counterflow to a flow of heat transferfluid traveling through the heat transfer fluid conduit.
 18. The deviceof claim 17 wherein the rotatable conveyor drum is an Archimedes screw.19. The device of claim 17 wherein when the rotatable conveyor drumrotates the granular material pours over the at least one heat transferfluid conduit such that heat exchange occurs between the granularmaterial and the heat transfer fluid.
 20. The device of claim 17 whereinthe rotatable conveyor drum is capable of rotating at one or morespeeds.